Field of the Invention
The present invention relates to heater used in semiconductor processing, and more specifically to a heater with multiple heater zones and thermocouples to monitor those zones.
Description of Related Art
In semiconductor manufacturing, silicon substrates (wafers) are processed at elevated temperatures for deposition of numerous different materials. Temperatures typically range in the 300-550 C range, but can at times go as high as 750 C or even higher. The deposited materials are “grown” in a layer on the surface of the wafer. Many of these materials have growth rates which are extremely sensitive to temperature, so variations of the temperature across the wafer can affect the local growth rate of the film, causing variations in the film thickness as it is grown across the wafer.
It is desired to control the variations in thickness of the deposited films. Sometimes it is desired to have the films thicker in the center of the wafer (like a dome). Sometimes it is desired to have the films thicker on the edge (like a crater or dimple). Sometimes it is desired to have the film thickness as even as possible (within tens of angstroms).
One of the most direct methods for controlling the temperature of the wafer, and thereby the thickness profile of the as-deposited films, is to place the wafer on a heater. By designing the heater with a specific watt-density “map” which produces the temperature profile desired on the wafer, the desired film thickness profile can be produced. Watt-density of the underlying heater is increased in the location(s) where higher temperatures are desired on the wafer, and decreased in the location(s) where lower wafer temperatures are desired.
It is desired by chip manufacturers to have the ability to run different processes in the same process chamber. Capital equipment for growing films is very expensive (more than $1 million per process chamber is typical), so it is desired to maximize the usage of, and minimize the number of required process chambers. Different temperature processes with different chemistries are run in the same chamber to produce different films. These different films may also have different growth-rate vs. temperature behavior. This leads the chip manufacturers to desire the ability to change the watt-density map of a heater in a given process chamber “on-the-fly” to achieve the desired film thickness profile.
Additionally, it is desired by chip manufacturers to have the ability to run exactly the same “recipe” in multiple process chambers and produce films that have matching film thickness profiles (as well as other properties which can be affected by temperature such as film stress, refractive index, and others). Therefore, it is desired to have the ability to produce a heater which can have very repeatable watt-density maps from unit to unit.
A heater can be made with the ability to change the watt-density map by using multiple independent heater circuits within the heater. By varying the voltages and currents applied to the different circuits, you can change the power levels in the locations of the individual circuits. The locations of these specific circuits are called “zones”. By increasing the voltage (and thereby the current as these heater elements are all resistance heaters) to a given zone, you increase the temperature in that zone. Conversely, when you decrease the voltage to a zone, you decrease the temperature in that zone. In this way, different watt-density maps can be produced by the same heater by changing the power to the individual zones.
At least two limitations have affected chip makers' ability to effectively use multi-zone heaters. The first limitation is that current state-of-the-art heaters have only one control thermocouple. Only one control thermocouple can be used because the plate-and-shaft design currently used for heaters allows for location of a thermocouple at the center of the heater plate only, or within a radius of ˜1 inch of the center of the heater. Thermocouples are made of metals which are incompatible with the processing environment of the wafer, and therefore must be isolated from that environment. Additionally, for fastest response of a thermocouple (TC) it is best to have it operating in an atmospheric pressure environment, not the vacuum environment of a typical process chamber. Therefore, TCs can only be located within the central hollow area of the heater shaft which is not in communication with the process environment. If there are heater zones located outside of the 2 inch diameter of the heater shaft, then no TC can be installed there to monitor and help control the temperature of that zone.
This limitation has been addressed by using “slaved” power ratios to control heater zones located outside of the central area of the heater. Ratios are established of the power to be applied to the central zone and to each of the other zones which produce the desired watt-density map. The central control TC monitors the temperature of the central zone, and the power applied to the central zone (which is based on the feedback of the central control TC) is then applied to all zones as adjusted by the pre-established ratios. For example, with a two-zone heater, let us assume that a ratio of 1.2 to 1.0 of power applied to the outer and inner zones produces the desired temperature profile. Let us assume that the heater control system, by reading the temperature data provided by the central control TC, determines that a voltage of 100VAC is needed to achieve the proper temperature. With the slaved ratio control methodology, a voltage of 120VAC will thereby be applied to the outer heater zone, and a voltage of 100VAC will be applied to the inner zone. The watt-density map can thereby be adjusted by changing the slave ratios.
This leads us to the second limitation. Current state-of-the-art heaters have an inherent variation of the resistance of the embedded heater(s). Due to the high temperatures and pressures required in the manufacturing process of current ceramic heaters, the resistance tolerance achievable can approach 50%. In other words, a typical resistance for a semiconductor-grade ceramic heater element is within a range of 1.8-3.0 ohms (at room temperature—the heater element material is typically molybdenum, which increases in resistance as the operating temperature increases).
This variation causes a problem with maintaining a repeatable watt-density-map from unit to unit with multi-zone heaters controlled by the slave-ratio method. With single zone heaters, the resistance variation may not be an issue, because a control TC is utilized to monitor the actual operating temperature, and power levels fed to the heater are adjusted accordingly. But if you have a multi-zone heater, and the heater element resistance variations can approach 50%, then the slave ratio control methodology will not produce a repeatable watt density map from unit to unit.
What is called for is to establish a heater design which will allow installation of multiple control TCs which can be physically located within the respective heater zones to allow for feedback and control directly, and yet still keep the TCs isolated from the processing environment within the process chamber.